The invention relates to the ex vivo destruction of contaminating tumor cells in stem cell transplants using intact bispecific antibodies.
With about 43,000 new cases/year, breast cancer occupies a top position in the cancer statistics of women in Germany. Less than one third of the women suffering from lymph node invasion at the time of diagnosis survive for 10 years without relapse.
Against this background, attempts have been made since several years towards the prolongation of life or even healing of female patients suffering from extensive lymph node invasion and distant metastases by means of autologous bone marrow and stem cell transplantation in connection with high-dose chemotherapy. Despite high response rates to the high-dose chemotherapy a permanent cure in the metastatic stage rarely occurs. Today, the therapy of metastasized mammary carcinoma is almost exclusively a palliative therapy. Clinical phase 1 studies were able to show that high-dose chemotherapy (HD-CT) followed by autologous bone marrow or stem cell trans-plantation (aPBSZT) is able to achieve complete remission in female patients with chemotherapy-sensitive metastasized mammary carcinoma. However, the remissions are limited in time and in most of the cases recidivism occurs. The recurrence may arise from clonogenic tumor cells which were reinfused by the transplant and/or from those which have survived the HD-CT in the patient. Micrometastases can be detected in bone marrow after chemotherapy using the sensitive RT-PCR for CK19 or ep-cam, respectively, for example C215, and by immunocytology. They are accompanied by an unfavorable prognosis even after HD-CT and autologous stem cell transplantation. Therefore, it seems neccessary to develop new concepts for the elimination of minimal residual disease (MRD) and for the purification of transplants from contaminating tumor cells. Presumably, upon achievement of a remission, the MRD consists mostly of tumor cells being resistant against antiproliferative chemotherapy by remaining in a resting state (kinetic resistance) or by developing biochemical mechanisms such as multi-drug resistance (MDR).
An essential problem in autologous stem cell transplantation is the contamination of the transplant by tumor cells which later may contribute to the occurence of a recidivism in the patient. To date mainly "purging" by immunomagnetic beads has been used for the purification of stem cell transplants from contaminating tumor cells. By this technique, tumor cells are caught on a magnet by bound antibodies carrying iron and, thus, removed from the transplant. However, the time involved and the high technical effort coupled to high costs (approx. 20,000.- DM/patient) are disadvantageous. Moreover, following this in vitro reduction of the number of residual tumor cells a recidivism still may occur--albeit after a delay. The reason for this may be the restriction of the method to the stem cell transplant as well as tumor cells escaping a mechanistic approach such as this for example by "aggregation with normal cells".
For these reasons, other immunological approaches to purify the stem cell transplant were tested such as the addition of activated T cells in combination with bispecific F(ab')2 fragments to redirect T cells to the tumor cells in vitro. It turned out that by such purging hematopoietic stem cells are not affected in their function--as measured in proliferation assays--but the ability to kill tumor cells in these experiments was relatively limited (1-2 log tumor reduction). Also, the use of preactivated T cells cultivated for two weeks has to be regarded as a disadvantage of this approach forcing up the effort and hampering clinical application.